Assuming it is a 7mm Rem. mag your stuck with a magnum case like a 6.5 Rem Mag, 264 Win Mag., or 26 Nosler unless you want to find another bolt that is a standard bolt face.
Doesnt the 26 nosler use a bigger bolt face than the win mag belted cases?
No, Ultra Mags still use a .540" bolt face, same as regular belted magnums.
Good to know... so technically, with a re-barrel, I could make my 7mag a 338 edge?